1.0 Field of the Invention
The present invention relates to a spectrometer for measuring atomic concentrations of an element within a substance and, more particularly, to a magneto-optic rotation (MOR) spectrometer having a high intensity laser source that improves the detection limits and the dynamic range of the (MOR) spectrometer.
2.0 Description of the Prior Art
Spectrometers employing spectroscopes are commonly employed in chemical analysis for the detection and measurement of atomic concentration of metals, such as alkali metals of sodium (Na) or potassium (k) in, for example, environmental samples or in shipboard fuels to detect for salt water intrusion. One traditional method of measuring atomic concentrations uses atomic absorption spectroscopy (AAS), herein referred to as an AAS spectrometer, more fully described, for example, in the text of A. Varma, Handbook of Atomic Absorption Analysis, Vol. 1 & 2 (CRC Press, 1984). In this technique, the sample to be tested is atomized, usually by aspirating into a flame or vaporized by a graphite furnace. Light from a hollow cathode lamp, serving as the light source of the AAS spectrometer, passes through the vapor where atoms absorb part of the light. According to the Beer-Lambert's law, the measured atomic absorption varies linearly with the atomic concentration of the element being analyzed. The detection of low concentrations by the AAS spectrometer is limited by its ability of measuring a small signal on top of a large signal representative of the background. Measuring concentrations larger than the upper limit for measuring by the AAS spectrometer requires dilution of the sample being measured, which increases the error in the measurement and also its measurement time. The dynamic range in AAS is defined by the difference or ratio of the lower and upper measurable ranges. Present AAS systems are mainly limited by their low dynamic ranges, .about.2 orders of magnitude, and sensitivity.
While AAS techniques for detecting alkali metals are well documented, this technique is actually too sensitive. For example, Na and K are easily measured in the part-per-billion range (ppb), however, part-per-million (ppm) samples must be diluted by three orders of magnitude to be in the linear range of the AAS spectrometer. These dilutions require large amounts of operator time and introduce error into the analysis. A technique which extends the useable dynamic range from the ppb to the ppm range would enhance the accuracy of the measurements and decrease greatly the amount of time required for the analysis. It is desired that a spectrometer be provided utilizing techniques that increase the detection limits and the dynamic range of detection.
Another disadvantage of an AAS spectrometer is the size and weight of its components. More particularly, AAS spectrometers require the use of 1/4 to 1/2 meter monochromators to separate the various wavelengths of light generated by its light source. These monochromators cause the AAS spectrometer to be too large for use on shipboard and aircraft platforms. It is desired that a spectrometer be provided that is free of monochromators. The elimination of the monochromators would result in a significant size and weight reduction and could potentially make the use of detecting metals in engine lubricants feasible onboard ships. Currently the engine lubricants are commonly sent off ship to be analyzed which create a long lag time for detecting potential problems. Furthermore, ships docking in port can't dump their water possibly containing contaminants until the water is analyzed. A man-portable atomic analysis method and related spectrometer would save considerable time and money in situations such as these.
An alternative method that is free of monochromators and that employs a Faraday or Voigt technique for measuring atomic concentrations and uses a magneto-optical rotation (MOR) effect is found in MOR spectrometers. The mechanism behind this MOR effect involving atomic vapors contained in vapor cells is Zeeman splitting of the atomic energy levels resulting from an applied magnetic field and is more fully described in the technical article entitled "Dispersive Magnetooptic Filters" of P. Yeh, published in Appl. Opt. 21, 2070 (1982). At a specific combination of optical frequency provided by the light source of the MOR spectrometer, pathlength, and atomic concentration, the polarization of plane-polarized light will be altered by the addition of a component which is rotated by .pi./2 radians. Cross polarizers, placed before and after the vapor cell, block all frequencies of light except for a very narrow band in the wings of the transition regions of the plane-polarized light that has been altered. The monochromators are not necessary in the Faraday or Voist technique utilized in MOR spectrometers since the cross-polarizers block all light which is not rotated, and, therefore, eliminate the need of the monochromators to separate the numerous spectrum lines generated by the light source of MOR spectrometers. Since the polarization change is very sensitive to atomic concentration, the transmission through the cross polarizers is well suited as an analytical tool for measuring unknown atomic concentrations.
MOR spectrometers are known and employ a hollow cathode lamp similar to those employed by AAS spectrometers. MOR spectrometers, although free of the relatively heavy monochrometers, have undesired detection limits and dynamic ranges similar to those of AAS spectrometers. It is desired to provide MOR spectrometers having an advantageous attendant light weight aspect but, in addition thereto, having improved detection limits and increased dynamic ranges.